Magna Doodle (thing)

Since its invention by Pilot Pen Corporation in 1974, millions of this popular toy were sold. This device, which we all remember and love, consists of a plastic board and a neat Stylus pen. Just draw whatever you like on the board and erase it later, with a twist of a dial.

The secret lays within the plastic board, which in fact is a Magnetophoretic display. The display is constructed of three layers -- a hexagonal plastic lattice in the middle, and two transparent plastic layers on the top and bottom. Each cell of the hexagonal lattice is filled with a thick liquid and many tiny magneticparticles.

The hexagonal lattice is required to achieve an equal distribution of magnetic particles across the display. The liquid within each cell has special properties which contradict the gravitational force applied on the magnetic particles, holding them in their place until they are pulled by a magnet.

As you might have guessed, the Stylus is equipped with a magnetic tip. The magnetic force of this tip must be strong enough, to pull the magnetic particles through the thick liquid, onto the top of the display.

To clear the display, you simply twist a dial. That dial is moving a magnetic stick, located under the display; as the stick moves across the board, its magnetic force pulls the magnetic particles to the bottom of the hexagonal lattice layer, and thus dissolving the drawing from the display.